The invention relates to a lossy triphase low-pass filter.
Commercially available voltage intermediate-circuit converters which are also referred to as frequency converters very often have a so-called output sinusoidal filter on the load side. This output sinusoidal filter is intended to be used to filter out the high-frequency components of each pulsed output voltage of this frequency converter in such a manner that only a fundamental voltage remains at a load, in particular an electric motor, connected to the load-side connections of the frequency converter. Such a sinusoidal filter usually consists of a filter inductor and a filter capacitor which are connected as an LC low-pass filter. These filter elements are tuned in such a manner that a cut-off frequency of this sinusoidal filter is between a maximum fundamental frequency and a switching frequency of the frequency converter.
LC low-pass filters have a resonant frequency. If such a filter is excited in the vicinity of this resonant frequency, this excitation is amplified very quickly. This excitation may be effected by mains harmonics or harmonics of a frequency converter connected to the mains.
On account of this problem, a low-pass filter is designed in such a manner that no excitation is effected in the vicinity of the resonant frequency. Since excitation cannot be entirely excluded, attenuation resistors which keep a resonant peak which arises within tolerable limits are provided as attenuation elements. However, an increased power loss must be expected with these attenuated low-pass filters.
These LC low-pass filters, in particular lossy low-pass filters, are used as mains filters or as output sinusoidal filters in frequency converters. Low-pass filters which have the problem described can be recognized by the fact that the root-mean-square value of a capacitor current in said filters becomes greater in the event of excitation at resonance than during normal operation.
FIG. 1 illustrates a load-side power converter 2, in particular an inverter, of a frequency converter (not illustrated in full) having an output sinusoidal filter 4 (FIG. 1 of EP 0 682 401 B1). In addition to the load-side power converter 2, an intermediate circuit 6 of the frequency converter is illustrated, said intermediate circuit having two capacitors 8 and 10 which are electrically connected in series. The junction point of these two capacitors 8 and 10 forms a so-called neutral point NP. The inputs of the output sinusoidal filter 4 are connected to the output-side connections 12, 14 and 16 of the frequency converter which are the AC-voltage-side connections of the load-side power converter 2. A lossy LC low-pass filter is provided as the output sinusoidal filter 4. This lossy triphase low-pass filter 4 has a filter inductor 18 and a filter capacitor 20 for each phase. These filter capacitors 20 are electrically star-connected. A junction point between a filter inductor 18 and a filter capacitor 20 respectively forms an output terminal 22, 24 and 26 of the output sinusoidal filter 4. A triphase load 28, in particular an electric motor, is connected to these output terminals 22, 24 and 26.
FIG. 2 illustrates a block diagram of a lossy triphase low-pass filter 30 of the generic type. This low-pass filter 30 is known from EP 0 682 402 B2, in particular from FIG. 1 of said patent specification. This lossy triphase low-pass filter 30 differs from the low-pass filter 4 according to FIG. 1 in that an attenuation element 32 is respectively electrically connected in series with a filter capacitor 20. A non-reactive resistor is respectively provided as the attenuation element 32. These attenuation elements 32 are used to attenuate the root-mean-square values of the capacitor currents in the event of excitation at resonance. The use of non-reactive resistors as attenuation elements 32 has the disadvantage that attenuation is less effective with little resonant excitation of the low-pass filter 30. With a large amount of excitation, not only the attenuation effect but also the attenuation losses increase quadratically with the capacitor current. These losses become greater, the greater the intended attenuation effect.